In this picture you can see three saints. On the left is Saint Mark, the patron saint of Venice. In the middle sits Saint Thomas Aquinas, and on the bottom right a child with long hair is kneeling. On the right is Saint Louis of Toulouse, wearing the clothes of a bishop.
The three saints do not have much in common. They lived at different times and therefore could not have met. But the three bear the first names of men from the family that commissioned and paid for the picture – Tommaso (Thomas) and Marco (Mark) from the Dragan family.
The Dragan family was very rich and important. They had invented a new way of making glass and were able to use it to produce particularly valuable crystal glass. Crystal glass was a luxury product at the time that only rich people could afford. Louis, on the right, is holding a piece of paper with a relevant Bible passage in his hand:
"Then he showed me a river of the water of life, clear as crystal, coming from the throne of God and of the Lamb."
But the painting tells another story: When Carpaccio showed the painting to the Dragan family for the first time, there was probably an incident. Carpaccio had painted a window with round panes of glass. These panes of glass are called bull's-eye panes. Bull's-eye pans were common and easy to make at the time.
But Carpaccio must have been mistaken: The Dragan family did not make bull's-eye pans, but gilded crystal vessels. Today it is assumed that the Dragan family rejected the painting and asked Carpaccio to paint over the bull's-eye panes.
You can see how he did it here: The angels holding the red ribbon and the small red angel heads further back were painted later. We know about this misunderstanding today because the bull's-eye panes were rediscovered by an infrared examination.